This invention relates to protective relaying apparatus of the type which utilizes phase comparison techniques. More particularly, this invention relates to such relaying apparatus wherein the transmitter keying threshold levels have been offset from the effective zero axis of the current waveform in each phase of a protected line section.
Normally, when a protected power line transmission line section is faulted, fault current will flow inwardly to the fault from energized busses. Under this type of fault condition, the relaying apparatus will cause breaker tripping to take one or more phases of the protected line section down until the fault is cleared. However, under certain internal fault conditions, as in the case of a high resistance line-to-ground or line-to-line, this will not occur. For example, assuming a strong power supply bus at one end of the protected line section and a weak power supply bus supplying load at the other end, it is possible to have a net outfeed from the protected line section into the weak bus. In such an instance, the relaying apparatus would not recognize nor respond to the internal fault and breaker tripping would have to be accomplished by secondary or backup protection.
A solution to this problem was set forth in the above-referenced U.S. patent application Ser. No. 548,135. In that case, the keying threshold levels of each transmitter, one being located at each end of the protected line section, were each offset a like and opposite amount from the effective zone axis of the current waveform. It was found, however, that this solution was satisfactory only where the magnitude of outfeed current is relatively small, that is, no more than approximately 4 amperes of secondary current.
This is due to the requirement that the keying threshold level be set above the maximum expected outfeed current. In some instances, this caused the keying threshold level to be set relatively high. While this arrangement proved generally satisfactory, it was found that the relaying apparatus would not respond or thereby trip an associated breaker in the case of an internal fault causing infeed current levels below that now recognizable by the relatively high keying threshold levels. In other words, offset keying with a relatively high threshold level led to a "blind spot" in the response of the relaying apparatus.